In a cell-based Code Division Multiple Access (CDMA) wireless environment, there may be more total users than codes available to allow multiple access to the communication channel. This shortage of capacity may require reuse of codes every few (spatially separated) cells. The spatial separation allows the communication signals using the same codes to decay before they have a chance to interfere with each other. However, because the number of users in a cellular network is typically highly variable, the codes that are allocated to a set of users of one cell need to be allocated and de-allocated based on the number of users of that cell. There are several issues that complicate the allocation of codes as the number of users of the system increase. Firstly, the total number of available codes may be finite. Secondly, overlap of codes between adjacent cells may not be allowed.
Thirdly, the RF environment may change unpredictably along with noise propagated through that environment. Because of the nature of CDMA, data encoded with some codes may be more susceptible to noise that other data, in a particular environment noise or multipath from code re-utilization may interfere during particular times. Therefore, an optimal cellular system should not allocate codes that may induce inordinately high bit errors until the number of users absolutely requires it.
In high-density multi-user ultra wideband environments, or in environments with high noise levels, unallocated CDMA codes (which might be allocated using pulse position modulation (PPM)) may be at risk of either having been allocated already or high in noise. In either case, in a dynamic CDMA environment, the system will then check for another unassigned code, followed by another and so on as necessary until an available code has been found.
This need to seek out and find a suitable unassigned codes increases proportionately as a function of increased number of users, increased channel capacity demands, and increased noise. In high-density, multi-user, high capacity consumption and/or noisy environments the availability of acceptable unassigned codes decreases. The resultant effect can mean greater demands on the system to search for suitable unassigned CDMA codes.
If one code is already in use, or subject to too much noise, or otherwise undesirable the system will simply search for another code. While this search is extremely rapid, taking only microseconds, if this search for optimum unassigned codes is frequent, or requires multiple searches for a good code, the demands on the system will result in inefficient management of available capacity.